1. Field of the Invention
The invention relates generally to wafer bonding of a wafer to a substrate.
2. Description of the Prior Art
There are several microelectronic and microwave devices that are formed on a silicon substrate that would benefit from the use of a high thermal conductivity, microwave insulating substrate instead of a silicon substrate. GaN High Electron Mobility Transistors (HEMT) are currently being grown on silicon wafers with a <111> wafer orientation, and have the feature of high electron mobility and transconductance. However, their performance in microwave circuit applications suffers because of microwave losses in transmission lines and the poor quality factor (“Q”) of inductors that are formed over the silicon substrate, since the silicon substrate is not obtainable as either insulating or semi-insulating. There are microwave losses in transmission lines and degraded quality factor even in the case that a high resistivity substrate in the range of 3000-10,000 ohm-cm resistivity is used. The power output capability from GaN HEMT transistors is strongly dependent on thermal conductivity of the substrate. The thermal conductivity of a silicon substrate is 140 W/mK. The other substrate that is typically used for GaN HEMT growth is a semi-insulating SiC substrate that has a thermal conductivity of approximately 300 W/mK. Semi-insulating SiC may be expensive and only available in small sizes.
Transmission line propagation issues and poor “Q” also exist for CMOS, BiCMOS, and SiGe Heterojunction Bipolar Transistor (HBT) circuits fabricated using silicon or Silicon-on-Insulator (SOI) substrates. High speed microprocessors suffer from enhanced propagation delay in the transmission of electrical signals on metal interconnects due to microwave loss issues in the non-insulating silicon substrate. In addition, wireless and mixed-signal RF circuits are frequently fabricated on silicon substrate. The “Q” of inductors in these wireless and mixed-signal circuits is degraded because these circuits are fabricated on a non-insulating silicon substrate. Silicon mixed signal circuits suffer from a problem of cross talk from the digital circuit to the analog circuit. It would be desirable to fabricate mixed-signal circuits on an insulating substrate to reduce cross talk effects. Silicon microprocessor circuits are currently becoming thermally limited. It would be desirable to implement silicon circuitry on a high thermal conductivity substrate.
There are a smaller number of applications, such as discrete microwave power transistors, where it is desirable to implement microwave transistors on a high thermal conductivity substrate. However, for these applications the substrate does not have to be insulating. The microwave transmission line matching circuits are typically implemented on a microwave board. The applications for discrete microwave transistors include microwave base stations, high power L- and S-band solid-state radars, cellular base stations, and C-band communication links. In the case of vertical current transport Si bipolar or SiGe HBT power transistors, it is desirable that the substrate be electrically conductive (and highly thermally conductive) in order make electrical contact to the collector.
High performance microwave transistors and circuits are often formed by epitaxial growth and device fabrication on GaAs, InP, or GaInAs substrates. The thermal conductivity of GaAs and InP substrates is relatively poor (on the order of 50-60 W/mK). The output power for a microwave transistor is often strongly dependent on the thermal conductivity of the substrate. Thus, it would be desirable to replace a significant portion of the GaAs or InP substrate with a high thermal conductivity, microwave insulating substrate.
There are four primary types of wafer bonding. These include 1) direct wafer bonding with atom-to-atom bonding, 2) direct wafer bonding with thin bonding material layer, 3) polymer wafer bonding, and 4) metal direct bonding. Standard packaging approaches also use a form of bonding to perform die attach. Typical die attach material include eutectics, epoxy, metal filled epoxy, ceramic filled epoxy, solder, phase change material, and silver glass. Die attach materials are typically greater than 25 microns thick. The general approach to improve the thermal conductivity of the die attach material is to use metal (typically silver) filled epoxy or ceramic (such as boron nitride) filled epoxy. The thermal conductivity of the ceramic filled epoxy is generally in the range of 1-10 W/mK.